Method for continuously casting a steel beam blank

ABSTRACT

A method for producing a steel beam blank includes continuously casting a steel beam blank strand and cooling the steel beam blank strand in a secondary cooling zone. The steel beam blank strand is guided in a vertical casting plane along a curved path having its web perpendicular to the vertical casting plane, so that each of the lateral flanges has an intrados flange tip and an extrados flange tip. The method further includes straightening the steel beam blank strand behind the secondary cooling zone. When being straightened, the intrados flange tips are selectively reheated between the secondary cooling and the straightening of the steel beam blank strand via an external energy supply focused on the intrados flange tips. In this manner, transverse cracks in the intrados flange tips may be reliably avoided.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for continuouslycasting a steel beam blank.

BACKGROUND OF THE INVENTION

[0002] Since the 1960s it is known in the art of steel making tocontinuously cast near-net-shape sections for rolling e.g. I-beams orH-beams. These near-net-shape sections are called beam blanks. They havea substantially H-shaped cross-section with a web centrally arrangedbetween two lateral flanges. Today such beam blanks are even used toroll Z-shaped sheet-piles and other steel sections.

[0003] Beam blanks are produced by continuous casting, i.e. liquid steelis continuously fed into a short, water-cooled copper mould with an openvertical casting channel, and a beam blank strand, which has the finalcross section of the beam blank to be produced, is continuouslywithdrawn from this mould. At the outlet of the continuous casting mouldthe continuous beam blank strand has only a thin solidified outer shellenveloping a liquid steel core. Solidification of the beam blank strandis then continued by spray cooling, wherein a cooling fluid, generallywater or an air-water-mist, is sprayed onto the perimeter surfaces ofthe beam blank strand. This spray cooling takes place in a secondarycooling zone beneath the continuous casting mould. In this secondarycooling zone the beam blank strand is guided in a vertical casting planealong a curved path, with its web being perpendicular to the verticalcasting plane. An extraction and straightening device, which is locateddownstream of the secondary cooling zone, straightens the bent beamblank strand, prior to pushing it onto a horizontal run-out table, wherebeam blanks of a desired length are cut from the continuous beam blankstrand.

[0004] It is well known in the art of continuous casting that a goodcontrol of the secondary cooling of the strand is of utmost importancefor the final quality of the cast product. It is indeed this secondarycooling that allows to control temperature evolution in the strandduring its final solidification, thereby allowing to control themicrostructure of the cast product.

[0005] While spray cooling in the secondary cooling zone of a continuouscasting line allows a rather good control of temperature evolutionduring solidification of billets, blooms or slabs, this is not yet thecase for beam blanks. Indeed, due to the fact that beam blanks have—incomparison with billets, blooms and slabs—a relatively complexcross-section (comprising elements of different thickness, orientationand perimeter surface to volume ratio), it is very difficult to closelycontrol the evolution of the temperature profile in a beam blank byspray cooling. A more or less uniform spray cooling of all the perimetersurfaces of the beam blank will for example inevitably result in anovercooling of the flanges. However, trying to avoid an overcooling ofthe flanges by reducing direct spray cooling of the flange surfaces,results in an insufficient cooling of the massive joining portionsbetween the flanges and the web, which still enclose important liquidsteel pockets. The consequence of an insufficient cooling of this liquidsteel pockets is a bulging of the shell in the flange/web joiningportions due to the internal pressure in the liquid steel pockets and anincreased risk of a liquid steel break-through. In conclusion,optimising the secondary cooling of a beam blank is a rather complexproblem, which has already been and still is the object of numerousresearch programs. However, despite the use of sophisticated computerprograms for selectively controlling spray cooling of the differentzones of the beam blank in function of various casting parameters,present beam blanks still tend to have major shortcomings.

[0006] One of these shortcomings of present beam blanks is the presenceof transverse cracks in the intrados flange tips. These transversecracks appear in the intrados flange tips when the beam blank isstraightened in the straightening device. They are observed inparticular, but not exclusively, in large section and high strength beamblanks. Although it is very likely that these transverse defects are dueto an undesired quench of the flange tips during secondary cooling, ithas not yet been possible to reliably avoid these cracks, e.g. by abetter control of the secondary spray cooling. In this context it has tobe pointed out that it is particularly problematic to control secondarycooling of the intrados flange tips, because these flange tips are notonly cooled by the cooling fluid that is directly sprayed onto theintrados portions of the flanges, but also by the cooling fluid that issprayed onto the intrados side of the web and of the web/flange joiningportions. Indeed, at least part of this intados cooling fluid flowslaterally over the intrados flange tips, thus causing an undesiredvigorous cooling of the latter. In order to reduce risk of quenching theflange tips, spray cooling of the intrados side of the beam blank strandshould therefore be generally limited, but this would result in otherproblems, as e.g. a bulging of the shell on the intrados side of theflange/web joining portions.

[0007] JP-A-10263752 is concerned with the prevention of warping at theflange part during the continuous casting of beam blanks. To preventthis flange warping this document suggests to eliminate the temperaturedifference between the flange surfaces and the last solidified part ofthe beam blank by heating the flange surfaces of the beam blank up to900-950° C. until the beam blank reaches final solidification or shortlythereafter. The Japanese document contains however no teaching aboutavoiding transverse cracks in the intrados flange tips during thestraightening of the beam blank.

OBJECT OF THE INVENTION

[0008] A technical problem underlying the present invention isconsequently to reliably avoid the formation of transverse cracks in theintrados flange tips during straightening of a beam blank whilenevertheless warranting a sufficient secondary cooling of the intradosside of the beam blank. This problem is solved by a method as claimed inclaim 1.

SUMMARY OF THE INVENTION

[0009] A method for producing a steel beam blank in accordance with thepresent invention comprises the known steps of:

[0010] continuously casting a steel beam blank strand with an H-shapedcross-section having a central web between two lateral flanges;

[0011] cooling the steel beam blank strand in a secondary cooling zone,wherein the steel beam blank strand is guided in a vertical castingplane along a curved path having its web perpendicular to the verticalcasting plane, so that each of the lateral flanges has an intradosflange tip and an extrados flange tip each of the intrados flange tipspresenting an intrados border surface;

[0012] straightening the steel beam blank strand behind the secondarycooling zones; and

[0013] providing an external energy supply to the flanges been thecooling in said secondary cooling zone and the straightening of thesteel beam blank strand.

[0014] In accordance with an important aspect of the present invention,the external energy supply is focused on each of the intrados flangetips, more particularly on a boundary zone immediately beneath theintrados border surface, so as to selectively reheat this boundary zonebefore the straightening of the steel beam blank stand. It has indeedbeen discovered that such a focused reheating allows to obtain aremarkable recovery of hot ductility of the steel in the flange tips,which is sufficient to reliably avoid the appearance of transversecracks during straightening of the beam blank strand. It will beappreciated in this context, that the method of the present inventionallows to design and optimise the secondary cooling of the intrados sideof the beam blank strand, without paying too much attention to a quenchof the flange tips. Indeed, In accordance with the present invention thenegative effects of such a quench of the flange tips are curedthereafter by means of the selective reheating of the flange tipsbetween the secondary cooling and the straightening of the steel beamblank strand.

[0015] In most cases it will be sufficient to determine the externalenergy supply so as to obtain reheating temperatures higher than 650°C., preferably higher than 800° C., in a boundary zone up to a depth of10 mm to 20 mm beneath an intrados border surface of the flange tip.This external energy supply, should further be determined so as not toexceed temperatures of 1000° C. within the intrados flange tips.

[0016] Furthermore, it is recommended that straightening of the beamblank shall take place when the reheated intrados flange tips still havetemperatures higher than 650° C., preferably higher than 700° C.

[0017] Viewed from the metallurgical point of view, it can be concludedthat it is of advantage if the external energy supply is determined soas to obtain, beneath the intrados border surface, a fine grainedferrite-pearlite structure with a thickness of about 10 mm to 20 mm.

[0018] The external energy supply is easily achieved by relativelysimple burner means comprising a plurality of burner nozzles alignedalong the intrados flange tips.

[0019] Induction heating necessitates more sophisticated equipment, butalso allows better control of the reheating operation. In case ofinduction heating, inductor means are arranged along the intrados flangetips, as to induce eddy currents in the intrados flange tips. Accordingto a first embodiment, the inductor means is located above the intradosborder surface and generates an alternating magnetic field penetratingthrough the intrados border surface into the flange tips. According to asecond embodiment, the inductor means defines an air gap, and theintrados flange tip is located within the air gap in a transversealternating magnetic field.

[0020] In order to achieve a good thermal efficiency of the reheatingoperation, it is recommended to carry it out under a heat insulatinghood.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:

[0022]FIG. 1: is a section through a continuous casting line with acurved secondary cooling path and a heating device located at the outletof the curved cooling path for selectively heating the intrados flangetips of the flanges of the beam blank prior to straightening the latter;

[0023]FIG. 2: is a section through the heating device of the continuouscasting line of FIG. 1, with a typical large section beam blank therein;

[0024]FIG. 3: is a schematic section showing a first type of anelectromagnetic inductor for selectively heating an intrados flange tipof a beam blank;

[0025]FIG. 4: is a schematic section showing a second type of anelectromagnetic inductor for selectively heating an intrados flange tipof a beam blank;

[0026]FIG. 5: is a transverse section showing the intrados half of abeam blank (the extrados half is not represented);

[0027]FIG. 6: is a photography of a transverse section through theterminal portion of the left beam blank flange, illustrating theboundaries between the different metallurgical structures in thissection (the area shown on the photography is identified by a dotedframe in FIG. 5);

[0028]FIG. 7: is a photography of a transverse section through theterminal portion of the right beam blank flange, illustrating theboundaries between the different metallurgical structures in thissection (the area shown on the photography is identified by a dotedframe in FIG. 5);

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0029] As best seen in FIG. 2, a typical steel beam blank, which is usede.g. for rolling e.g. I-beams or H-beams, but also for rolling Z-shapedsheet piles, has a substantially H-shaped cross-section, with a web 14that is centrally arranged between two lateral flanges 16′, 16″. Massivejoining portions 18′, 18″ connect the web 14 to the lateral flanges 16′,16″.

[0030]FIG. 1 shows a continuous casting line 10 for producing such steelbeam blanks using a process in accordance with the present invention. Ina manner known per se, a refractory-lined liquid steel distributor 20,generally called tundish, continuously feeds liquid steel into a short,water-cooled casting mould 22 with an open vertical casting channel 23.A beam blank strand 24 is continuously withdrawn from this casting mould22. At the outlet of the continuous casting mould 22, the beam blankstrand 24 has a thin solidified outer shell, which already has the finalform of the beam blank to be produced, but still has liquid steelpockets therein.

[0031] Solidification of the beam blank strand 24 is then continued byspray cooling, wherein a cooling fluid, generally water or an air-watermist, is sprayed onto the perimeter surfaces of the beam blank strand24. (It will be noted that the wording “spray cooling” used hereincovers classical spray cooling as well as the so-called “air mistcooling”.) This spray cooling takes place in a secondary cooling zone 26beneath the continuous casting mould 22. Herein, the beam blank strand24 is guided along a curved path in a vertical casting plane (i.e. theplane of FIG. 1). In the continuous casting line 10 shown in FIG. 1, thesecondary cooling zone 26 consist of four guiding and spray coolingsegments 26 ₁, 26 ₂, 26 ₃ and 26 ₄. Each of these guiding and coolingsegments 26 ₁ . . . 26 ₄ comprises a plurality of guiding and supportrollers 27 and spray means (not represented). The guiding and supportrollers 27 co-operate to define the curved path for the beam blankstrand 24.

[0032] Referring now to FIG. 2, wherein the vertical casting planecontaining the curved centreline of the beam blank 24 is indicated witha doted line 27, it is seen that the curved beam blank strand 24 has itsweb 14 perpendicular to the vertical casting plane 27. Consequently,each of the two flanges 16′, 16″ of the curved beam blank strand 24 hasan intrados flange tip 28′, 28″ and an extrados flange tip 30′, 30″. Theintrados side of the curved beam blank strand 24 is hereinafteridentified with reference number 32, and its extrados side withreference number 34.

[0033] Referring back to FIG. 1, it will be noted that reference number38 globally identifies an extraction and straightening unit, comprisinge.g. four extractors 38 ₁, 38 ₂, 38 ₃, 38 ₄ which straighten the bentbeam blank strand 24 and finally guide it onto a horizontal run-outtable 40. On this run-out table 40, oxyacetylene torches 42 cut out beamblanks of a desired length of the continuous beam blank strand 24. Aheating device 44 is arranged between the secondary cooling zone 26 andthe extraction and straightening unit 38. In accordance with the methodof the present invention, this heating device 44 is used to selectivelyheat the intrados flange tips 28′, 28″ of the curved beam blank strand24 before the latter is straightened in the extraction and straighteningunit 38.

[0034] Before describing in greater detail preferred embodiments of theheating device 44, the effects, characteristics and advantages of thisselective heating of the intrados flange tips 28′, 28″ of the flanges16′, 16″ will now be described, inter alia with reference to FIGS. 5, 6and 7.

[0035] It has been discovered that during the spray cooling of the beamblank strand 24 in the secondary cooling zone 26, the intrados flangetips 28′, 28″ are not only cooled by the cooling fluid that is directlysprayed onto the intrados portions of the flanges 16′, 16″, but also bythe cooling fluid that is sprayed onto the intrados side 32 of the web14 and of the web/flange joining portions 18′, 18″. Indeed, at leastpart of this cooling fluid flows laterally over the intrados flange tips28′, 28″, thus causing an undesired vigorous cooling of the latter, withthe result that they have a quenched microstructure when the beam blankstrand 24 leaves the secondary cooling zone 26. In FIGS. 6 and 7, lines50′ and 50″ indicate the boundary between a quenched microstructure zone52′, 52″ above the lines 50′, 50″ and an equiaxed ferrite-pearlitemicrostructure 54′, 54″ below the lines 50′ and 50″.

[0036] At the outlet of the secondary cooling zone 26, the quenchedmicrostructure zones 52′, 52″ extend from the lines 50′, 50″ to theintrados border surfaces 56′, 56″ of the intrados flange tips 28′, 28″,and the temperatures in these zones are generally in the range of 550°C. to 650° C. It has been discovered that, in this temperature range,the residual ductility of the steel in the quenched zones of theintrados flange tips 28′, 28″ is particularly low, which explains theappearance of transverse cracks in the intrados flange tips 28′, 28″during the subsequent straightening of the beam blank strand 24.

[0037] In accordance with the present invention, the intrados flangetips 28′, 28″ are selectively reheated to temperatures higher than 650°C., preferably higher than 800° C., prior to the straightening of thebeam blank strand 24. It will be appreciated that with a reheating ofthe flange tips 28′, 28″ to temperatures in the range of 650° C.-750°C., i.e. a temperature range generally still too low to achieve asignificant transformation of the quenched microstructure into aferrite-pearlite microstructure, an already remarkable recovery of hotductility can be observed. If a reheating of the flange tips 28′, 28″ totemperatures in the range of 750° C. to 900° C. is achieved, atransformation of the quenched microstructure into a ferrite-pearlitemicrostructure takes place. For lower temperatures in this range, thetransformation of the quenched microstructure is only partial, but withhigher temperatures it gets more and more complete, until a finenormalised ferrite-pearlite microstructure is finally obtained.Reheating of the flange tips to temperatures higher than 1000° C. shouldbe avoided, because such high temperatures favour an undesired graingrowth.

[0038] In FIGS. 6 and 7, the lines 58′, 58″ indicate the boundarybetween the original quenched microstructure zone 52′, 52″ at the outletof the secondary cooling zone 26 and a zone 60′, 60″ in which reheatinghas transformed the quenched microstructure in a fineferrite-pearlite+acicular ferrite microstructure. It will be noted thatthe zones 60′, 60″ have near the outer edge 62′, 62″ of the flange 16′,16″ only a thickness of about 10 mm to 20 mm, i.e. only about 30% to 40%of the thickness of the quenched zone 52′, 52″ in this zone. Experiencehas indeed shown that for preventing transverse cracks in an intradosflange tip 28′, 28″ during straightening of the beam blank strand 24 itis already sufficient if only an intrados boundary zone of 10 mm to 20mm recovers a good hot ductility before straightening of the beam blankstrand 24. In other words, the quenched zone 52′, 52″ below the heattreated zone 60′, 60″ may maintain a relatively low ductility withoutgenerating transverse cracks in the intrados flange tips 28′, 28″ duringstraightening of the beam blank strand 24. This result may be explainedin that a thin ductile outer shell 60′, 60″, which extends to the outeredge 62′, 62″ of the flange 16′, 16″, is sufficient to prevent initialstarting of transverse cracks. This knowledge is rather important,because it allows to conclude that the external energy supply should inparticular be focused on the outer edge 62′, 62″ of the intrados flangetip 28′, 28″, and that the useful penetration depth of the heattreatment need only be of 10 mm to 20 mm. Consequently, only arelatively small heating capacity is required, and surface temperaturescan be maintained below 1000° C.

[0039] Referring now to FIG. 2 a first embodiment of the heating device44 will be described. This heating device 44 comprises a heat insulatinghood 80, which is provided with a refractory lining 81 and covers theintrados side 32 of the beam blank strand 24. Two gas burner rails 82′,82″—one for reheating the left intrados flange tip 28′ and one forreheating the right intrados flange tip 28″—are integrated in this hood80. Each of these gas burner rails 82′, 82″ comprises a plurality ofburner nozzles 84′, 84″, which are aligned along the intrados flange tip28′, 28″ and designed so as to focus their flames onto the intradosborder surface 56′, 56″ near the outer edge of the respective flange tip28′, 28″.

[0040]FIG. 3 and FIG. 4 illustrate inductive heating of an intradosflange tip 28′. In the embodiment of FIG. 3 the flange tip 28′ isarranged in an air gap 90 of a water-cooled electromagnetic inductor 92,which generates an alternating magnetic field 94 that is substantiallyparallel to the intrados border surface 56′ of the flange tip 28′. Thisalternating magnetic field induces eddy currents in the flange tip 28′located in the air gap 90, causing this flange tip to be reheated. Inthe embodiment of FIG. 4, a water-cooled electromagnetic inductor 96 isarranged parallel to the intrados border surface 56′ of the flange tip28′. Water cooled conductors 98 generate an alternating magnetic field100 that penetrates through the intrados border surface 56′ into theflange tip 28′, causing it to become heated. Heat conduction warrants adeeper penetration of the thermal energy produced by the eddy currentswithin a small boundary layer under the intrados border surface 56′ ofthe flange. Depending on the temperature to be reached and the magneticproperties of the steel of the beam blank (inter alia its Curie point),it may be necessary to subdivide the electromagnetic inductor 92, 96into several units, each unit being supplied with a current of differentfrequency, so as to achieve different penetration depths.

[0041] The heating device 44 should preferably be arranged between thesecondary cooling zone 26 and the extracting and straightening unit 38;i.e. before the first extractor 38 ₁. If however, in an existing castingline, there is not sufficient place before the first extractor 38 ₁, itis also possible to arrange the heating device 44 between the firstextractor 38 ₁ and the second extractor 38 ₂, respectively to divide itinto two units, one being arranged before the first extractor 38 ₁, theother being arranged between the first extractor 38 ₁ and the secondextractor 38 ₂. It is of course also possible to arrange a heating unitupstream of each extractor 38 ₁.

1-10 (canceled).
 11. A method for producing a steel beam blank,comprising steps of: continuously casting a steel beam blank strand withan H-shaped cross-section having a central web between two lateralflanges; cooling said steel beam blank strand in a secondary coolingzone, wherein said steel beam blank strand is guided in a verticalcasting plane along a curved path having its web perpendicular to saidvertical casting plane, so that each of said lateral flanges has anintrados flange tip and an extrados flange tip, each of said intradosflange tips presenting an intrados border surface; straightening saidsteel beam blank strand behind said secondary cooling zone; andproviding an external energy supply to said flanges between said coolingin said secondary cooling zone and said straightening of said steel beamblank strand, wherein said external energy supply is focused on each ofsaid intrados flange tips on a boundary zone immediately beneath saidintrados border surface, so as to selectively reheat said boundary zonebefore said straightening of said steel beam blank strand.
 12. Themethod as claimed in claim 11, wherein: said external energy supply isdetermined so as to obtain reheating temperatures higher than 650° C.,preferably higher than 800° C., in said boundary zone up to a depth of10 mm to 20 mm beneath said intrados border surface.
 13. The method asclaimed in claim 12, wherein: said external energy supply is determinedso as not to exceed temperatures of 1000° C. within said reheatedboundary zones.
 14. The method as claimed in claim 12, wherein: saidstraightening of said beam blank takes place when said reheated boundaryzones still have temperatures higher than 650° C.
 15. The method asclaimed in claim 11, wherein: said external energy supply is determinedso as to obtain in said reheated boundary zones a fine grainedferrite-pearlite structure with a thickness of about 10 mm to 20 mm. 16.The method as claimed in claim 11, wherein: said external energy supplyis achieved by burner means comprising a plurality of burner nozzlesaligned along said intrados flange tips.
 17. The method as claimed inclaim 11, wherein: said external energy supply is achieved by inductormeans arranged along said intrados flange tips, said inductor meansinducing eddy currents in said intrados flange tips.
 18. The method asclaimed in claim 17, wherein: said inductor means is located above saidintrados border surface and generates an alternating magnetic fieldpenetrating through said intrados border surface into said flange tips.19. The method as claimed in claim 17, wherein: said inductor meansdefines an air gap, and said intrados flange tip is located within saidair gap in a transverse alternating magnetic field.
 20. The method asclaimed in claim 11, wherein: said selective heating of said boundaryzones takes place under a heat insulating hood.